1. Field of the Invention
This invention is directed to an improved polyester film having excellent slip properties and the process for producing said film. Still more specifically, the instant invention is directed to a polyethylene terephthalate film coated with a latex coating which imparts excellent slip properties and the process for producing the film.
2. Summary of the Prior Art
The growth of microfilm and magnetic tape markets has significantly increased the utilization of polyester film as supports for those products. However, polyester film becomes electrostatically charged, especially at low relative humidity. This electrostatic charge attracts not only dust and other contaminants but, additionally, attracts other polyester film supports. For instance, electrostatic forces interfere in the collating, sorting and developing of microfiche cards by causing the cards to stick together.
To overcome this serious problem, antistatic polyester films have been developed. Antistatic polyester films are usually formed by the application of an antistatic coating onto the surface of the polyester film. Many of these coatings have successfully reduced the electrostatic properties of the polyester film to satisfactorily low levels. However, many of these coatings cause blocking. That is, the polyester film coated with an antistatic layer has a coefficient of friction high enough so that one layer of the film does not slip over another or over rollers and other equipment over which the film moves during operation.
Another problem arising from the coating of an antistatic layer onto a surface of a polyester film is the decreased clarity resulting therefrom. This property, usually reported as haze, is very important in microfilm applications. Microfilm must be readable. Light is projected through the film so that the viewer may read the information printed on the film. Poor clarity results in fuzziness and difficulty in reading because of loss of light intensity.
The above discussion indicates the importance of polyester film having good slip, or handleability and clarity. Although the prior art includes disclosures directed to antistatic coating on polyester films none of them produce films having the combination of good antistatic, slip and clarity properties.
U.S. Pat. No. 4,089,997 issued to Van Paesschen et al. includes a disclosure of stearamidopropyldimethyl-.beta.-hydroxy-ethylammonium nitrate. However, the disclosure in the '997 patent includes additional constituents, different from those in the instant invention, which do not produce the excellent slip and clarity properties.
Not only are the constituents present in the coating critical to the film's properties but, moreover, the concentrations of these materials are similarly of great importance. For instance, U.S. Pat. No. 3,264,136 issued to Hedge, discloses a coating which includes 0.2 percent by volume of stearamidopropyldimethyl-.beta.-hydroxy-ethylammonium nitrate in combination with the additional constituent, polymethylmethacrylate. This combination of constituents did not produce the combination of antistatic, slip and clarity properties necessary for utilization in polyester film supported microfilm and magnetic tapes.
An additional requirement in film technology is not directed to the properties of the film substrate. It is not enough that a latex coating provide the important properties discussed above. The latex coating must be stable. That is, the latex coating cannot precipitate out on standing, to form a two-phase system, for at least one week. This property, satisfactory pot life, is necessary for commercial operation of a coating process. One can ill afford to devise a coating which does not stay emulsified long enough to coat a suitable length of film.